Conventionally, the in vivo SPF is used as a scale representing the ultraviolet radiation protection effect of external preparations for skin for preventing sunburn due to ultraviolet radiation, such as sun protection products. This SPF, which is an index indicating the effect of skin protection from sunburn due to ultraviolet radiation and sunburn prevention, is defined by the value obtained by dividing the amount of ultraviolet radiation necessary for causing slight redness in the case of using an external preparation for skin by the amount of ultraviolet radiation necessary for causing slight redness in the case of not using an external preparation for skin. For example, sunburn is ten times less likely to be caused with use of a sun protection product of in vivo SPF 10 than in the case of bare skin.
The in vivo SPF may be measured by exposing each of skin protected by an external preparation for skin and unprotected skin to a certain amount of ultraviolet radiation and determining the next day whether sunburn (erythema) has been caused using artificial light very close to sunlight (a solar simulator).
Use of the in vivo SPF makes it possible to objectively evaluate the ultraviolet radiation protection effect of external preparations for skin. However, measuring the in vivo SPF necessitates the cooperation of a large number of volunteers of specific skin types, thus requiring a large amount of money and a large number of days.
Therefore, Patent Documents 1 through 3 disclose in vitro SPF evaluation methods that measure estimated in vitro SPFs without using volunteers. Further, a polyethylene sheet, a nylon film (see Patent Document 4), a quartz plate, a PMMA plate (see Non-Patent Documents 1 and 2), etc., are known as skin substitute membranes used for in vivo SPF evaluation methods. On a surface of one side of the nylon film of Patent Document 4, grooves with V-shaped vertical sections in a shorter-side direction, imitating sulci cutes, are provided, and irregularities are provided in the part other than the sulci cutes by blasting.
On the other hand, Non-Patent Document 3 determines that the amount of an external preparation for skin applied in measuring in vivo SPFs should be 2.00 mg/cm2. However, no known skin substitute membrane allows an external preparation for skin to be evenly applied with such an amount of application. In the case of using a known skin substitute membrane, the amount of application of the skin substitute membrane in measuring estimated in vitro SPFs is approximately 0.75 mg/cm2 to approximately 1.20 mg/cm2.
Further, when a material subject to degradation due to ultraviolet radiation is used as an ultraviolet absorber contained in an external preparation for skin, the degradation of the ultraviolet absorber progresses at the time of measuring in vivo SPFs. There is doubt as to whether conditions for measuring in vivo SPFs are sufficiently reproduced in the estimated in vitro SPFs of an external preparation for skin containing such a ultraviolet absorber because the amount of application of the external preparation for skin is different from that in the case of measuring in vivo SPFs. Here, causing the amount of application of the external preparation for skin in measuring estimated in vitro SPFs to be 2.00 mg/cm2, which is the same as in the case of measuring in vivo SPFs, is important in reproducing a condition for measuring in vivo SPFs for not only an attenuation pattern in the case of degradation caused by ultraviolet radiation but also the condition of application of the external preparation for skin at a microscopic level.    [Patent Document 1] Japanese Patent No. 3337832    [Patent Document 2] Japanese Laid-Open Patent Application No. 2008-96151    [Patent Document 3] Japanese Laid-Open Patent Application No. 2008-111834    [Patent Document 4] Japanese Laid-Open Patent Application No. 2002-48789    [Non-Patent Document 1] Ferrero, L. et al.; “Importance of Substrate Roughness for In Vitro Sun Protection Assessment,” IFSCC Magazine, Vol. 9, No. 2, 97-108 (2006)    [Non-Patent Document 2] COLIPA GUIDELINES, METHOD FOR THE IN VITRO DETERMINATION OF UVA PROTECTION PROVIDED BY SUNSCREEN PRODUCTS, Edition of 2007    [Non-Patent Document 3] International Sun Protection Factor Test Method, (COLIPA, JCIA, CTFA SA, CTFA), May 2006